Biodiesel is becoming increasingly useful as a biodegradable, nontoxic diesel fuel (Ma and Hanna, Bioresource Technology 1999, 70, 1-15). Examples of biodiesel include soy diesel (methyl soyate), rapeseed methyl ester, and various vegetable and animal fat methyl esters. Biodiesel fatty acid methyl esters (FAME) have been recently accepted as a viable alternative to traditional petroleum-derived solvents that are of environmental concern and are under legislative pressure to be replaced by biodegradable substitutes that result in reduced environmental impact. Although interest in biodiesel is rapidly increasing, the process by which it is synthesized has not substantially changed in recent years.
Soy diesel is currently prepared commercially by an energy and labor intensive process wherein soybean oil is reacted with methanol at elevated temperature (often 140-150° F.), and often under elevated pressure, in the presence of sodium methoxide as a homogeneous catalyst. This process is called “transesterification”. Isolation of the desired methyl soyate from the highly caustic (toxic) catalyst and other products, such as glycerol, involves a precise neutralization process with strong acids, such as hydrochloric acid (HCl), and extensive washes with water to remove the resulting sodium chloride (NaCl) salt. Also, glycerol must be separated from the sodium chloride salt by vacuum distillation. Because glycerol has a significantly high boiling point, the distillation becomes a costly and energy intensive operation (see Bender, M., Bioresource Technology 1999, 70, 81; Diasakou et al., Fuel 1998, 77, 1297; Ogoshi and Miyawaki, J. Am. Oil Chem. Soc. 1985, 62, 331; Suppes et al., J. Am. Oil Chem. Soc. 2001, 78, 139).
Current biodiesel preparation processes do not allow the catalyst to be recycled, due to the high solubility of sodium methoxide in methanol. Additionally, the labor and materials required for the neutralization, separation, and removal of the catalyst creates economic and environmental concerns. To circumvent these issues, researchers worldwide have been developing solid catalysts for the transesterification of oils to biodiesel. For example, various basic metal oxides, such as magnesium methoxide, calcium oxide, calcium alkoxide, and barium hydroxide, have been demonstrated to be active catalysts for transesterification (Gryglewicz, S., Applied Catalysis, A: General 2000, 192 (1), 23-28). However, these solid base catalysts have little or no recyclability due at least in part to the solubility of the solid metal oxides and hydroxides in methanol (Gryglewicz, S., Bioresource Technology 1999, 70 (3), 249-253).
Accordingly, there is a need for efficient, inexpensive, and environmentally friendly catalysts for biodiesel production that do not have the solubility, separation, and recyclability problems associated with currently known catalysts. There is also a need for new methods for efficient, inexpensive, and environmentally friendly biodiesel production that do not have the problems that are associated with the currently known methods.